Adsorption and Diffusion of Hydrogen on Nickel

Abstract

The surface diffusion and adsorption of hydrogen on Ni field emitters has been investigated by a technique previously used for similar studies of H and O diffusion on W. It was found that diffusion occurs with an activation energy of 7±1 kcal in the chemisorbed layer, without the precipitation boundaries observed on W. This is believed to result from the inherent homogeneity of f.c.c. substrates, on which most adsorption sites seem equivalent. This is supported by the marked lack of emission anisotropy of hydrogen‐covered Ni field emitters. A field dependent transformation in the adsorbed film was found between 2° and 4°K and interpreted as a shift in the equilibrium between adsorbed H₂ molecules and the terminal fraction of adsorbed H atoms. The implications of this result for the mechanism of H₂–D₂ exchange are discussed. The activation energy of desorption at very low coverage is determined to be 46±4 kcal from the temperature dependence of desorption rates.